MergePlugin uses project source not jar source

Update gradle MergePlugin to use the 'project.sourceSets.main.output'
for jar content rather than 'project.jar.source'. This prevents
superfluous MANIFEST.MF files from appearing in the merged jar.

Issue: SPR-10324

buildSrc/src/main/groovy/org/springframework/build/gradle/MergePlugin.groovy

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2013 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -110,7 +110,7 @@ class MergePlugin implements Plugin<Project> {
 
 		// update 'into' project artifacts to contain the source artifact contents
 		project.merge.into.sourcesJar.from(project.sourcesJar.source)
-		project.merge.into.jar.from(project.jar.source)
+		project.merge.into.jar.from(project.sourceSets.main.output)
 		project.merge.into.javadoc {
 			source += project.javadoc.source
 			classpath += project.javadoc.classpath